1. Field of the Invention
The present invention relates to composite materials, particularly curable prepregs and semipregs, particularly those which find use in industrial applications particularly in wind energy applications such as wind turbine blades. In particular the invention is concerned with providing prepregs and semipregs of high fibre area weight which can be processed in automatic processing machines and which do not require hand lay-up when being moulded and cured. The invention further relates to a method of producing such curable prepregs and semipregs and the processes for the manufacture of articles there from and the articles produced particularly articles having a high fibre loading.
2. Description of Related Art
Composite materials have well-documented advantages over traditional construction materials, particularly in providing excellent mechanical properties at very low material densities. As a result, the use of such materials is becoming increasingly widespread and their fields of application range from “industrial” and “sports and leisure” to high performance aerospace components. Fibre reinforced cured resins are examples of such materials with the nature and type of the fibre, the amount of fibre and the nature of the resin being chosen according to the application. Industrial applications requiring high strength can require high fibre loadings which can make processing in automatic processing machines difficult if not impossible,
Prepregs, comprising a fibre arrangement impregnated with resin such as epoxy resin, are widely used in the generation of such composite materials. Typically a number of plies of such prepregs are “laid-up” as desired and the resulting laminate is cured, typically by exposure to elevated temperatures, to produce a cured composite laminate. Automated processes are available for the laying up and curing of such prepregs.
A particular type of prepreg is the so-called semipreg, which involves the fibre arrangement being only partly impregnated with resin, leaving a portion of the fibre arrangement in a “dry” state.
Semipregs can provide lower porosities in the final cured composite material, as the dry regions allow a pathway for entrapped air to escape from the laminate. This is particularly important in wind energy applications, where cost restrictions generally mean that curing takes place out of autoclave and at lower pressures.
In typical automated lay-up machines that lay-up prepregs or semipregs the prepregs or semipregs are automatically fed to a position where they are placed in a stack and condensed by the application of heat and pressure. In order for the stack to be adequately formed under automatic processing conditions it is important that the prepregs or semipregs do not adhere to the feeding equipment and do not form deposits of fibrous debris on the feeding equipment. At the same time the prepregs or semipregs must contain sufficient resin to allow impregnation of the fibres during curing and also provide sufficient fibre to provide the strength required in the finished cured article. To date this has been difficult generally and particularly difficult in the processing of high fibre content semipregs and prepregs required for the production of high strength industrial materials such as wind turbine structures such as blades and spars therefore.
A common semipreg arrangement is to have a layer of curable resin in contact with one or two adjacent layers of fibres which stay essentially dry with only very little resin migrating into the adjacent fibres. Such semipregs find particular use as part of large load-bearing structures e.g. spars for wind turbine blades. Another common arrangement is for the curable resin to be in contact with one adjacent layer of essentially dry fibres and another adjacent layer of resin-impregnated fibres. At high fibre loadings in industrial processing it has been necessary to lay-up the layers by hand.
A common fibre arrangement is woven, biaxial or triaxial, as such overlapping and interaction of the fibres helps to retain the integrity of the semipreg. In wind energy applications it is common for the fibre layers to be heavy weight, such as 600 gsm per layer or more typically from 600 gsm to 1500 gsm. However such semipregs are typically laid down by hand, as they are difficult to process in known types of automated laying apparatus. This is because such equipment relies of a degree of adhesion between the plies in the stack to be present, so that the material being processed can be laid down and adhere to the surface below. Semipregs are dry on their exterior and so seem not be capable of automated lay-up, by known methods as they lack the necessary surface tack. Furthermore, if prepregs or semipregs are employed with sufficient surface tack they can adhere to and form deposits on the automatic processing equipment.
Additionally, such semipregs, particularly the heavy weight variants employed in wind turbine structures, tend to produce excessive fuzz from broken fibres which can build-up in any automated apparatus and cause problems. This is particularly the case with woven or other overlapping fibre arrangements.
It would therefore be highly desirable to develop a method of automatically laying down a semipreg, particularly the heavier weight variants employed in industrial applications such as wind energy structures.